Agents for immunizing or vaccinating animals include a large variety of different solutions to various problems involved in immunization.
When immunizing with small antigens or haptens, one problem is often the absence of peptide sequences in the immunogen that can trigger CD4+ cells to provide the necessary help for B-cell or CTLs to mount a sufficient immune response. This problem is especially important when the antigen is of self-origin.
The traditional approach has been to covalently couple (conjugate) the antigen of interest to a large immunogenic carrier protein. In recent years this technology has been refined by utilising immunogens comprised of short peptide antigens fused to relevant carrier proteins or even to known universal T-helper epitopes.
However, both of these prior art technologies suffer a number of drawbacks. If using the traditional carrier approach, it is difficult to characterise the resulting product and it is further a problem that the immune response induced will have a tendency to be directed towards the carrier moiety and not to towards the peptide antigen—this phenomenon is known as “carrier suppression”.
Furthermore, when coupling a small peptide to a large carrier, it is normally not predictable whether or not the antigenic determinant(s) of the peptide will be accessible to the immune system, since the carrier moiety to some degree will shield the peptide.
Similar problems exist with fusion of carriers to peptides—this problem is to some extent avoided when using “carriers” that are constituted of short T-helper epitopes. Nevertheless, this may also give rise to problems. Due to the small size of both the peptide and the T-helper epitope, it is not in any way a certain thing that the ratio between T-helper epitopes and antigenic determinant is optimal in order to trigger the most effective immune response.
In conclusion, there is a definite need to device novel strategies for preparing vaccine based on small antigenic peptides or small haptens.